A web-offset printing press operates to print a multi-color image by combining several single color images through superimposed printing on a moving substrate or web. A typical four color printing process includes black, cyan, magenta and yellow ink. The color quality of the printed image is determined by the degree to which the colors of the printed image match a desired or exemplary reference image, which is often provided or endorsed by the print customer. One way to evaluate color involves visual examination of the printed image by a trained pressman. Another way to evaluate color is to measure the optical density of a solid color bar printed on the substrate. In general terms, the actual color quality is compared to the desired quality, and the amount of ink fed to the substrate is adjusted as necessary.
In particular, the printing press includes an inking assembly for each color of ink used in the printing process. Each inking assembly includes an ink reservoir as well as a blade disposed along the outer surface of an ink fountain roller. The amount of ink supplied to the roller train of the press and ultimately to a substrate such as paper is adjusted by changing the spacing between the edge of the blade and the outer surface of the ink fountain roller. The blade is divided into a plurality of blade segments, and the position of each blade segment relative to the ink fountain roller is independently adjustable by movement of an adjusting screw, or ink key, to thereby control the amount of ink fed to a corresponding strip or zone of the substrate extending in the longitudinal direction. A typical printing press includes 24-30 ink keys which operate to control ink to an ink key zone having a dimension of approximately 1.2-2.5 inches.
Ink is also spread laterally from one ink key zone to adjacent zones on the substrate due to the movement of vibrator rollers, which oscillate in a lateral direction relative to the longitudinal direction of travel of the substrate.
In order to preset the initial positions of the ink keys, it is common for a printing press operator to visually examine printed copies or proofs of the image to be printed and to note the amount of color necessary in respective zones of the image to be printed. Based on this visual examination as well as experience with the press, ink, and type of substrate (typically paper), the operator may preset the ink keys to approximate the settings that will be required once the press is running. As an example, low-tack yellow ink has a low pigment strength and requires a greater amount of ink to produce an image with a given optical density. As another example, uncoated paper requires more ink than does coated paper to achieve an image having a given optical density.
Once the printing press is started, the rate of ink flow from the ink fountain to the web must be controlled by adjusting the ink keys for each of the ink colors. The time spent for the ink key adjustment until the desired solid ink density for each zone is achieved on press is termed makeready. Again, ink key adjustment is typically achieved based on visual examination and manual adjustment by an experienced press operator. After makeready, it is common for a press operator to continually monitor the printed output and to make appropriate ink key adjustments in order to achieve appropriate quality control of the color of the printed image. For example, if the color in a zone is too weak, the operator adjusts the corresponding ink key to allow more ink flow to that zone; if the color is too strong, the corresponding ink key is adjusted to decrease the ink flow. Also during runtime, further color adjustments may be necessary to compensate for changing press conditions, or to account for the personal preferences of the customer.
The above-described visual inspection and manual adjustment techniques used in connection with ink key presetting, makeready, and runtime are relatively inaccurate, expensive, and time-consuming. Additionally, such techniques require a high level of operator expertise.
Methods other than visual inspection of the printed image are known for monitoring color quality once the press is running. These methods typically include measuring the optical density of a printed image. Optical density of various points of a printed image can be measured by using a densitometer or scanning densitometer either off-line or on-line of the web printing process. Optical density measurements are performed by illuminating a test image with a light source and measuring the intensity of the light reflected from the image. Optical density (D) is defined as: EQU D=-log.sub.10 (R)
where R is the reflectance, or ratio of reflected light intensity to incident light intensity.
Since substrate material is wasted until acceptable color is achieved, an accurate and quick method of determining ink key settings will minimize the required time and material costs. Especially for print jobs of short duration, start-up waste can be a major percentage of total time and materials required.
Typically, a conventional proportional-integral-derivative (PID) controller is the most widely used controller in industry. A PID controller is a control system where the control signal is a weighted sum of the current error, the summation of past errors, and the change in error since the previous sampling. The error is defined as the difference between the measured value and a target value. The weights are selected to provide the desired system performance. In particular, it may be beneficial to set one or two of the weights to zero.
The conventional PID controller was developed in the 1940's based on the classical linear time-invariant system. Theoretically, such a controller would work well in a printing application to control ink feed rate provided that the entire printing process was linear and time invariant. In other words, for example, the color density would need to be proportional to the ink key settings and the factors affecting the entire printing process would need to remain unchanged.